The present invention relates generally to alternating current (AC) motors and, more particularly, to a system and method for determining stator winding resistance for thermal protection of AC motors.
Thermal protection is an important aspect in the monitoring of motor conditions, as motor failures can often be related to thermal stress on stator winding insulation. It is commonly assumed that the motor's life is reduced by 50% for every 10° C. increase in temperature above an acceptable stator winding temperature limit.
Thermal protection of AC motors is important not only to running motors, but also to de-energized motors. In many applications, the AC motor is periodically de-energized to allow the motor to cool down prior to the next start. Also, overload relays may be used to trip the AC motor to protect the motor windings if the motor overheats. If the motor is tripped by overload relays, a certain amount of time is typically required before the motor can be restarted. Either this recovery time may be too conservative and production time is lost, or the recovery time may be too short and the incomplete cooling accumulates after each shutdown, potentially leading to premature damage to the winding insulation due to overheating.
Overheat protection of the stator winding insulation of AC motors is only one aspect of thermal protection. When electric machines are shut down, the stator winding temperature may fall below the ambient temperature, causing moisture condensation on the stator windings, brushes, and other compartments. This condensation can be detrimental to the life of the motor in certain applications. To avoid the moisture condensation or accumulation, motor winding pre-heating can be desirable to maintain the stator winding temperature above the ambient temperature.
Various methods and mechanisms for determining the stator winding temperature are currently employed for thermal protection purposes. Aside from the direct stator winding temperature measurement, thermal model-based and motor parameter-based temperature estimation methods are two techniques for thermal protection. The thermal model-based methods estimate the stator winding temperature using motor thermal models. However, due to the thermal parameter variation and the difficulty of thermal parameter identification, the accuracy of these methods may fall outside acceptable ranges. Besides, due to possible changes in cooling conditions, the thermal parameters are not always constant, and may need to be identified for each motor under each specific cooling condition.
Also, even if a thermal model or temperature measurement is determined for a given motor, existing stator winding heating devices heat the motor using two phases of the stator windings, allowing a single current flow path in the stator winding. This, however, leaves one phase unheated, or reliant on inductive heat. Also, because the stator resistance is relatively small, a large voltage and current input is typically needed to heat the motor. This large voltage and current input may reduce the life of the stator winding.
Because an AC motor may sustain damage if the stator winding temperature is outside an acceptable range or if the stator windings are heated at too high of a voltage and current input, accurate monitoring and controlling of the stator winding temperature in a de-energized AC motor is beneficial for motor protection purposes.
It would therefore be desirable to design an accurate, non-intrusive method for monitoring and controlling stator winding temperature in a de-energized AC motor, in an efficient manner and without adding further resistance to the motor.